1. Field of the Invention
The present invention relates to an optical instrumentation device for use in experiments involving high-pressure shock waves generated by impacts or explosives. More specifically, the present invention relates to a fiber optic probe for focusing laser light onto a specimen and for collecting reflected laser light from the specimen for the purpose of determining the specimen's surface velocity history during high-pressure shock wave experiments.
2. The Description of Related Art
Shock experiments involving projectile impacts or explosive detonations have been used for several decades to determine material properties under dynamic conditions and at extremely high pressures. A review of this field can be found in the paper by L. C. Chhabildas and R. A. Graham, "Developments in Measurement Techniques for Shock-Loaded Solids," in Techniques and Theory of Stress Measurements for Shock Wave Applications, Edited by R. B. Stout, F. R. Norwood, and M. E. Fourney, American Society of Mechanical Engineers, AMD - Vol. 83, pp 1-18 (1988). One of the most valuable instrumentation techniques in shock experiments has been laser velocity interferometry, in which laser light is focused onto the specimen's diffusely reflecting surface. Some of the reflected light is collected, and, as the surface moves during a shock experiment, the doppler shift of the reflected light is measured in an interferometer. The continuous measurement of the doppler shift results in a continuous velocity history of the surface of the shocked specimen, from which, together with other information, the specimen material properties are calculated, as described in the above-mentioned paper by Chhabildas and Graham.
In traditional experiments, the light from the laser, located remote from the impact site, is directed by mirrors to the impact chamber, where it is focused by a lens at a point on the specimen surface. The same lens collects some of the reflected light, collimating it for the return trip to the interferometer, usually located close to the laser. One difficulty with this approach is that intense laser beams are directed through work areas, creating problems for eye safety. To address this concern, work areas should be restricted during shot set-up, resulting in loss of worker efficiency. Another difficulty concerns the complexity of the optical alignment when a number of mirrors are used to direct the laser beams to and from the specimen in the impact chamber.
Attempts have been made to address these difficulties by using optical fibers to transport the laser light to the specimen and from the specimen back to the interferometer. The use of optical fibers requires lens elements close to the specimen to focus the light from a fiber onto the specimen surface and to collect reflected light into a fiber leading to the interferometer. The assembly comprising the fiber ends and the lenses held in the correct positions with respect to each other has been called a fiber optic probe. Because a fiber optic probe is normally located close to the specimen surface, in most experiments it is impacted and destroyed by the specimen just after it serves its function.
One of the first uses of fiber optic probes for shock wave instrumentation was described by Durand of France in a paper entitled "The Use of Optical Fibers for Velocity Measurement by Doppler-Laser Interferometry with Fabry-Perot Interferometer," published in the French journal Journees de Detonique in 1984. Durand's version of the fiber optic probe made use of a single fiber to carry the laser light both to and away from the shocked specimen. Using a single fiber has two disadvantages: First, about 4% of the light from the laser is reflected back into the fiber by the glass surface of the fiber's end. This can easily be more light than is collected into the fiber from the diffusely reflecting specimen surface. However, the internally reflected light from the fiber's end contains no doppler shift information, and thus it is detrimental to the velocity measurement. Second, the fiber supplying light to the specimen is the same size as the fiber collecting the reflected light, since they are one and the same fiber. However, it is an advantage to have a small diameter fiber to deliver light to the specimen so it can be focused to a small point, but a much larger fiber for collecting the reflected light to enhance its light-gathering aperture. Thus, Durand's fiber optic probe had shortcomings both in purity of the signal light returned to the interferometer and in light collection efficiency.
Another fiber optic probe design was described by S. Gidon, G. Garcin, and H. Behar, also of France, in their paper "Doppler Laser Interferometry with Light Transmission by Two Optical Fibers," which appears in the Proceedings of the 16th International Conference on High Speed Photography and Photonics, held in Strasbourg, France, Aug. 27-30, 1984. Although Gidon, et. al. solved the fiber end reflection problem by using a second fiber for the return light, both fibers were the same size, and no adjustment provisions were made to position the return fiber's end at the point of focus of the reflected light. Thus, the reflected light collection efficiency was very poor in their design.
Within the last four years, I personally designed two versions of the fiber optic probe while I was still employed at Sandia National Laboratories. Neither of these designs has been described in the literature. In the first design, a 2 mm diameter graded index (GRIN) rod lens was used to focus the light from a fiber with a 50 .mu.m core diameter onto the specimen surface, and a second 2 mm diameter GRIN lens, adjacent to the first, was used to collect reflected light into a second fiber with a 200 .mu.m core leading to the interferometer. The use of two fibers solved the internal reflection problem of single-fiber probes, but the probes were quite inefficient at gathering reflected light, their depth of field (distance the specimen surface could move before appreciable loss of light occurred) was quite small, their alignment mechanism was difficult to use, and they were expensive to make.
In my second design, a 2 mm hole was drilled through the center of a 10 mm diameter, 10 mm focal length glass lens, and a GRIN rod lens was cemented in the hole to capture the light from a 50 .mu.m fiber and focus it onto the specimen surface about 20 mm away. The 10 mm diameter lens then captured reflected light and concentrated it onto the end of a 300 .mu.m fiber leading to the interferometer. Although this design gathered more reflected light than my first design, the amount of light returned to the interferometer was still marginal, the depth of field was small, the alignment of the optics of the probe was difficult, and the probe was expensive to fabricate.